A basic assumption in the traditional twin method which compares within pair differences in monozygotic and dizygotic twins is that environmental variances in the twin types are equal. Strictly speaking, this method cannot be applied if there is evidence that the total variance differs between MZ and DZ twins. Twin research at Indiana University has demonstrated an association of twin type with the means and variances of a number of quantitative biological traits, and that maternal effects constitute an important source of variance heterogeneity. It appears therefore that environmentally caused inequality in the variances may obscure genetic influence inferred from twins, once thought to provide the strongest evidence for genetic control of dental traits. From a unique panel of adult twins from known placentation, tooth size and a number of dental morphological traits will be examined in three types of twins: dizygotic, monochorionic monozygotic and dichorionic monozygotic. Total, among pair and within pair mean squares as well as the group means will be compared. Where the means differ, further genetic analysis becomes unwarranted. Where total variances differ, we seek to revise presently available but biased estimates of genetic variance and heritablity by using combined among pair and within pair estimates. The two subtypes of monozygotic twins will be compared to examine prenatal maternal environment as a possible source of variance heterogeneity. Animal studies have consistently demonstrated striking evidences for maternal influence on tooth size. The monochorionic and dichorionic MZ twin model represents the first study designed to search for maternal influences on tooth morphology in man.